In the production of glass objects, particularly the mass production of bottles and jars, on a continuous basis, it is of considerable importance to be able to control the characteristics of the glass as it leaves the furnace and moves towards an appropriate forming unit, for example an independent section container forming machine of known type.
It is well appreciated that major problems can arise in terms of consistency of manufacture if great care is not taken to operate under homogeneous and effectively invariant conditions. The problem, however, is that the attainment of such conditions is particularly difficult to achieve.
Conventionally, glass is manufactured by charging the raw materials into one end of an essentially elongate furnace while applying heat, for example from oil burners, to heat the raw materials and fuse them together to form a glassy mass. The glassy mass moves away from the ingredient feed point, down the furnace, gradually becoming more and more homogeneous. It then flows, very slowly since it is very viscous, into a number of channels, perhaps as many as six or eight in a large furnace, known as forehearths and at the end of each channel there is located in the floor of the channel an aperture through which molten glass is discharged.
By ensuring a sufficient length of forehearth, and by applying appropriate insulation and/or heating, it is possible to ensure that, by the time the glass reaches the end of the forehearth and is ready for discharge, it is relatively free of bubbles and has a relatively stable internal pattern of temperature and viscosity. It is not, however, easy to ensure that discharge from the forehearth is always even.
A major problem arises from the fact that the glass, when discharged, must be in a highly viscous state, so that it may be moulded and, at the same time, cooled such that as it is moulded to its final shape, it has cooled to a temperature at which it is effectively sufficiently rigid to maintain that shape during subsequent cooling and annealing steps. Because the glass on discharge to the forehearth is highly viscous, and because, in any event, it moves very slowly, attempts to control the precise temperature and homogeneity of the emergent glass are fraught with difficulty.
In order to be able to control the overall flow of glass from the forehearth, it is a known practice to provide located above the aperture or apertures in the forehearth through which molten glass may pass, a metering cylinder which may be lowered on to the forehearth floor to prevent glass flow, and may be raised to permit glass to flow under its edge and through the aperture or apertures in the forehearth floor. In operation, such a cylinder may be rotated about its axis when it has been raised a little way from the forehearth floor, and such rotation can assist in homogenising the glass, but only to a limited extent. Alternatively, the cylinder may be stationary, and other stirring members moved in the glass to promote an homogeneous glass condition within the region above the aperture(s). Actual discharge of the molten glass through the aperture is conventionally achieved by the use of one or more vertically reciprocating plungers above the aperture(s) which act to form successive gobs of glass below the aperture which are cut off by synchronised shears to fall into a chute and be transported to a forming station.
Such a system is disclosed, for example, in U.S. Pat. No. 3,133,803, and, in addition, in that case flow may be adjusted by the use of a vertically-adjustable skimmer block set in the roof of the forehearth and which acts as a gate under which molten glass flows before running across a shallow land and down an inclined wall set to one side of a well into which the glass then flows and which surrounds the metering cylinder and gob plunger, and in which a shallow layer of molten glass forms. The glass is spread out in an attempt to render it more easily heatable or coolable, but such spreading out leads to problems of uneven flow and possible bubble entrainment.
Such an approach using a thin glass layer is unconventional, and has not been adopted widely in practice. In contrast, conventionally, the depth of glass in a forehearth is 100 to 160 mm, this depth being a compromise between making the forehearth sufficiently shallow that the temperature of the glass passing through it can be quickly controlled with little thermal lag, and making the forehearth deep enough to enable sufficient glass to flow along it. The discharge end of the forehearth is conventionally even deeper, for example 200 to 350 mm, giving a reservoir of glass of supposedly even characteristics from which gobs are successively drawn.